1. Field of the Invention
This invention relates to computer graphics systems and, more particularly, to arrangements for cueing the viewer that images presented on computer displays involve a third dimension.
2. History of the Prior Art
In presenting three dimensional representations of objects on two dimensional output displays, some means must be used for indicating the existence of the third dimension to the viewer. Various methods of accomplishing this result have evolved from standard drafting techniques. An example of this is perspective drawing in which parallel straight lines defining an object converge as the object recedes from view.
Certain other techniques have developed because of the peculiarities of the computer display. For example, in a bit-mapped computer output display which is presented on a cathode ray tube (CRT), a method used to indicate or cue the viewer to the different depths of portions of objects is to vary the intensity of the bits defining the object as the depth of the portion of the object increases. Thus, for a line running from the viewer's position to infinity, the closest location of the line is given the fullest intensity and the farthest location a zero intensity. The fading of the line in intensity provides the same effect as that given by images as they extend progressively further from the viewer. This technique enhances the feeling of depth of the line on the output display.
The prior art method of accomplishing this effect requires that each pixel of a line having a depth dimension have a different intensity value from a maximum to zero. In actual systems of the prior art, any individual line having a depth dimension is drawn by dividing the line length by the number of pixels and calculating an intensity value for each adjoining pixel of which the line is composed. Thereafter, each individual pixel is written to the output display, a single pixel at a time. In color systems in which the color of the line varies from one end to the other, the circuitry of such systems must calculate intensities for each of the colors red, green, and blue separately, each from zero to one hundred percent intensity. The prior art arrangements for accomplishing this form of depth cueing for each pixel require a separate calculation engine for each of the colors. All of these operations are very time consuming.
In improving computer systems, the emphasis has been on improving the speed of operation of such systems. This is especially true of systems using bit-mapped displays where the graphics output is usually the point of the system which limits its speed. Consequently, providing depth cueing in three dimensional bit-mapped graphics output systems has been speed limiting for the system.
Moreover, certain computer graphics systems utilize a technique called anti-aliasing for improving the apparent rendering of a line on a display. In such a technique, various pixels defining a line are rendered at different intensities to remove the sharp jagged edges along the sides of the line. Antialiasing is a technique well known in the art and is described, for example, in "The Aliasing Problem in Computer-Synthesized Shaded Images," Franklin Crow, March 1976, UTEC-CSC-76-015, ARPA Report. In such systems, providing depth cueing is not the straight forward process of varying line intensity because line intensity variations are already a part of the system for providing the anti-aliasing effect.
It is, therefore, an object of the present invention to improve the speed of computer graphics systems.
It is another object of the present invention to provide an extremely rapid means of providing depth cueing in a computer graphics system.
An additional object of this invention is to provide a new technique for providing depth cueing in a three dimensional computer graphics system.
Yet another object of this invention is to provide a system capable of both anti-aliasing and depth cueing.